Redundant switched reluctance motor

ABSTRACT

A multiple phase switched reluctance motor (30). A stator assembly (32) has a plurality of inwardly salient teeth (34a-341) terminating at a central bore (36). A rotor assembly (38) is disposed for rotation in the bore and has a plurality of rotor teeth (42a-42d). The stator assembly has at least one redundant pole set for each motor phase. The rotor assembly has a corresponding proportional number of rotor teeth. The redundancy of the poles helps distribute ovalizing forces on the motor assembly. This lessens the effect of these forces and reduces motor noise produced by the assembly in response to the forces. In addition, the stator can have multiple teeth per pole and the rotor a multiple of the determined rotor teeth. The number of rotor teeth exceeds the number of stator teeth so the rotor teeth overlap the stator teeth. This improves starting torque in a desired direction of motor rotation.

BACKGROUND OF THE INVENTION

This invention relates to switched reluctance motors and, moreparticularly, to a switched reluctance motor having redundant poles toreduce the "ovalizing" effects on the motor's structure and the noisegenerated thereby.

In U.S. patent application 747,855, which is assigned to the sameassignee as the present application, there is disclosed a shifted pole,single phase variable reluctance motor. One of the problems addressed bythe invention disclosed in this co-pending application is noisegenerated by motor "ovalizing". This problem arises because the motor'sstator and rotor assemblies are usually installed in the same frame. Asa result, mechanical forces are created within the assembly as the rotorand stator poles come into and go out of alignment. Thus, when polesalign, the assembly is subjected to an inward (pulling) force. When thepoles reach 90 degrees out of alignment, an outward (pushing) force isproduced. The consequent distortion changes the normal circular (incross-section) motor structure into an oval shape. Flexure of the motorstructure, caused by this distortion, produces noise which can reachundesirably high levels.

The structure disclosed in the co-pending application helps alleviatethis problem in a single phase, unidirectional motor. The problem,however, exists for other type motors as well. In this regard, otherapproaches to solving the ovalizing problem have been used. See, forexample, U. S. Pat. No. 4,998,052. The present invention advances thesolution to the ovalizing problem addressed by applicant's own previouswork and that disclosed in this '052 patent so that a solution to thisproblem is now available for poly-phase, bi-directional switchedreluctance motors as well as for variable reluctance motor's of the typedescribed in the co-pending application.

SUMMARY OF THE INVENTION

Among the several objects of the present invention may be noted theprovision of a poly-phase, bi-directional, general purpose switchedreluctance motor; the provision of such a motor in which the effects ofovalizing are substantially reduced; the provision of such a motor whichis an N-pole motor and which has redundant pole sets for each motorphase; the provision of such pole set redundancy to minimize the forcescreated and exerted on a motor assembly when the rotor and stator polesalign and when they are 90 degrees out of alignment; the provision ofsuch a motor in which the stator is constructed without cantileveredstator teeth thereby reducing motor noise; and, the provision of such amotor having a rotor tooth arrangement which facilitates starting themotor in a desired direction.

In accordance with the invention, generally stated, a multiple phaseswitched reluctance motor assembly comprises a stator assembly having aplurality of inwardly salient teeth terminating at a central bore. Arotor assembly is disposed for rotation in the central bore and also hasa plurality of teeth. The stator assembly has at least one redundantpole set for each motor phase and the rotor has a correspondingproportional number of opposed teeth. The redundancy of the polesdistributes the ovalizing forces on the motor assembly to lessen theeffect of these forces and reduce motor noise produced by the assemblyin response to such forces. Other objects and features will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of the stator and rotor of a three-phasemotor having two poles per phase;

FIG. 2 is an elevational view of the stator and rotor for a three-phasemotor similar to that of FIG. 1 but with redundant poles in accordancewith the teachings of the present invention;

FIG. 3 is an elevational view of a prior art three-phase motor havingcantilevered stator teeth;

FIGS. 3A and 3B are elevational views of three-phase motors made inaccordance with the present invention, each motor having four poles perphase and two teeth per pole;

FIG. 4 is an elevational view of a four-phase motor made in accordancewith the present invention and having four poles per phase and two teethper pole;

FIG. 5 is an elevational view of a three-phase motor made in accordancewith the present invention and having six poles per phase and two teethper pole; and,

FIGS. 6A and 6B respectively illustrate the improvement in reducingovalizing in a conventional motor (FIG. 6A) as compared with a motorconstructed in accordance with the present invention (FIG. 6B).

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, a switched reluctance motor is indicatedgenerally 10 in the drawings. The motor has a stator assembly 12 whichincludes a plurality of inwardly salient teeth, for example, the sixteeth 14a-14f shown in FIG. 1. The teeth terminate at a central bore 16in which a rotor assembly 18 is disposed for rotation. Rotor 18 ismounted for rotation on a central shaft 20 and includes a plurality ofopposed teeth, two such teeth 22a-22b being shown in the drawings. Motor10 is a three-phase motor (phases A, B, and C as indicated in FIG. 1)having six poles, The stator teeth 14a, 14d are associated with onephase, teeth 14b, 14e with a second phase, and teeth 14c, 14f with thethird phase. Stator coils (not shown for drawing clarity) are locatedbetween the stator teeth and each tooth is activated by the coils whenthey are energized. Further, each rotor tooth has first section 24adefining an air gap between the rotor and stator assemblies. To eitherside of section 24a are step gap sections 24b, 24c respectively. Each ofthese sections defines a wider air gap between the stator and rotor andfacilitates starting the motor in either direction by increasing thetorque in the desired direction of rotation. These stepped air gaps alsoserve to distribute normal forces which helps reduce ovalization.

Referring to FIG. 6A, motor 10 is again shown. The dashed line Xrepresents the distortion of the motor structure produced by the"ovalizing" effect on the motor. As seen in FIG. 6A, rotor teeth 22a,22b are aligned with the phase A teeth 14a, 14d. When the teeth arealigned as shown, an inward, or compressive force is produced on themotor assembly at the aligned pole positions. An outward force isconcurrently produced at the other poles. As the rotor teeth sweeparound bore 16, consecutively aligning with the phase B and phase Cpoles, the forces on the motor represented by line X follow them. Sincethe motor typically rotates at thousands of revolutions per minute (rpm)there is a constant flexing of the motor structure. This flexureproduces an undesirable, high level noise which needs to be reduced oreliminated. It will be understood that this ovalizing effect occursirrespective of the direction of motor rotation.

Referring to FIG. 2, a poly-phase, bi-directional switched reluctancemotor of the present invention is indicated generally 30. This motor hasa stator assembly 32 which includes a plurality of inwardly salientteeth, there now being twelve such teeth 34a-341. These teeth terminateat a central bore 36 in which a rotor assembly 38 is disposed forrotation. As before, the rotor assembly is mounted for rotation on acentral shaft 40. Whereas rotor 18 had two teeth, rotor assembly 38 hasfour teeth 42a-42d, these being evenly spaced about the rotor. Motor 30,like motor 10 is a three-phase motor (the phases A, B, and C beingindicated in FIG. 2). It is an important aspect of this invention thatstator assembly 32 has at least one redundant pole set for each motorphase. The redundancy of the poles helps distribute the ovalizing forceson the motor assembly, this distribution lessening the effect of theforces on the assembly. This, in turn, reduces the motor noise producedby the assembly in response to such forces.

For the stator assembly configuration of FIG. 2, it is also importantthat rotor 38 have a number of teeth which is proportional to the numberof stator teeth, including the teeth for the redundant poles. Motor 30has 12 stator poles, six of which are redundant. Each stator pole hasone associated stator tooth. The relationship between the number ofstator teeth and rotor teeth is given by the equation: ##EQU1## where phis the number of motor phases. Thus, since motor 30 is a three phasemotor having 12 stator poles and 12 stator teeth, the number of rotorteeth are the 4 teeth 42a-42d shown in FIG. 2.

Referring to FIG. 6B, the redundant pole construction of the presentinvention is important for the normal operation of the motor in that itreduces the ovalizing effect on the motor. Whereas in the motor 10configuration, there is a peak force (inwardly or outwardly) every 60degrees, in motor 30, this occurs every 30 degrees. The intensity of theforces exerted on motor 30 can be as great as those exerted on motor 10.However, since these maximum forces occur closer together, their effectis more distributed around the motor assembly. Thus, as shown in FIG.6B, while there is an ovalizing force still acting on the motorassembly, the distribution produced by the additional, or redundant,poles is seen to substantially lessen the flexing of the motor assembly.As a result, the motor noise resulting from the constant flexure issignificantly less.

As with rotor 18 of motor 10, teeth 42a-42d of rotor 38 each has a firstsection 46a defining an first air gap with respect to the statorassembly 42. Each tooth further has a second section 46b on one side ofsection 46a. Section 42b defines a second and larger air gap withrespect to the stator assembly. Each tooth further has a third section46c on the opposite side of the section 46a, with the air gap defined bysection 46c corresponding to that defined by second section 46b. Therespective step gaps produced by section 46b, 46c improve startingtorque for starting the motor in a preferred direction of rotation.

It will be understood that while three phase motor 30 has one redundantstator pole for each stator pole of three phase motor 10; and, inaccordance with the above given formula, 4 rotor teeth, the motor couldhave additional redundant poles. Thus, for example, the motor could have18, or 24, or 30 poles. According to the formula, for a three phasemotor, the rotor would have 6, 8, or 10 teeth respectively, if there isone stator tooth for each stator pole. Where the ratio between statorand rotor teeth is thus 6:2, 12:4, 18:6, etc., for a three phase motor;for a four phase motor, the respective ratios are 8:2, 16:4, 24:6, etc.The more redundant stator poles and rotor teeth which can be designedinto a motor, the more the ovalizing forces are distributed and the lesstheir effect. Accordingly, the lower the noise caused by thisphenomenon. As a practical matter, the limiting factor on the number ofredundant poles are space limitations of the motor; for example, thespace between adjacent stator teeth needed to insert coils into thestator assembly.

Referring to FIG. 3, a prior art three-phase motor M is depicted whichhas a rotor R and a stator S. The stator of motor M has multiple teeth Tfor each stator pole P. The motor also has a redundancy of six poles. Asis conventional in this prior art motor, the stator teeth arecantilevered off of each of the twelve poles. In operation, when thecompression or normal forces act on these teeth, they flex inwardly.This produces noise. And, the more the teeth are cantilevered, the moresusceptible they are to movement, and to noise generation.

Referring now to FIG. 3A, a motor 50 is shown in which there are aplurality of stator teeth for each stator pole. Motor 50 is a threephase motor and first includes a stator assembly 52 having 12 statorpoles 54a-541, six of which are redundant poles. Now, however, eachstator pole has two associated teeth. The stator teeth are teeth56a-56x. The stator assembly has a central bore 58 in which a rotorassembly 60 mounted on a shaft 62 is disposed for rotation. Becausemotor 50 has two teeth per stator pole, the number of rotor teeth,calculated in accordance with the previous formula, is 8. However,unlike the rotor construction of the previous embodiment of the presentinvention, rotor assembly 60 has a multiple number of the calculatednumber of rotor teeth. As shown in FIG. 3, the rotor has assembly hasfour times the number of calculated teeth, or 32 rotor teeth 64a-64af.It should be noted that the stator poles and teeth of motor 50 eliminatethe cantilever design of motor M, thereby to reduce motor noise.

The rotor teeth of motor 50 are spaced so that they overlap a statorpole. This is done to increase the amount of starting torque in thedirection of desired motor rotation For motor 50, the spacing betweenrotor teeth is given by the formula: ##EQU2## where: 360 is the numberof degrees in a circle,

N is the number of rotor teeth,

ph is the number of phases,

p/ph is the number of poles per phase, and

0.35 is the optimum ratio of tooth width to tooth pitch.

For motor 50, there are 3 phases, 4, poles per phase, and 32 rotorteeth. Substituting these values into the above formula yields: ##EQU3##for Which the calculated spacing is 14.81 degrees Using theapproximation formula: ##EQU4## provides a calculated spacing of 15.00degrees.

In addition, the proper number of rotor teeth is determined inaccordance with the formula; i.e.: ##EQU5##

Referring now to FIG. 3B, motor 50' is a three phase motor with fourpoles per phase.

This motor is similar to motor 50 except that instead of having 32 rotorteeth, motor 50' has 28 rotor teeth 64a-64ab'. Applying the previouslygiven formulas to the conditions of motor 50',

N=28

ph=3

p/ph=4, and ##EQU6## which satisfies this condition.

Next, ##EQU7## using the approximation, ##EQU8## 12.64 is notapproximately equal to 15.7.

What this means is that motor 50' will work, but the copper space willbe less than preferable. As a practical design consideration, thereshould be a 50/50 ratio between the space available for copper and thetooth space. This condition is satisfied, for example, in motor 50 ofFIG. 3A.

As a practical matter, the number of rotor teeth is again a function ofthe ability to fabricate a rotor assembly with a large number of teeth.In a larger size motor it may be more practical to have a largermultiple of rotor teeth than in a smaller size motor. Clearly, thespacing between the rotor teeth will be a function of the number ofteeth.

Referring to FIG. 4, a four phase motor 70 has a stator assembly 72 with16 poles 74a-74p. As with motor 50, each stator pole has with two teethper each pole. Consequently there are 32 stator teeth, these beingdesignated 76a-76af respectively. The motor further has a rotor assembly78 disposed for rotation in a central bore 80 of the stator assembly,and having forty rotor teeth 82a-82ao. Using the formula givenpreviously: ##EQU9##

As with motor 50, motor 70 has a multiple of the required number ofrotor teeth; for example 40 rotor teeth 82a-82ao respectively. Applyingthe spacing formula set forth above: ##EQU10##

The calculated spacing between stator teeth is now 10.35 degrees, andthe approximation is 11.25 degrees. Note that the calculated spacing isless than the approximation, which is acceptable. If, for example, motor70 had 44 rotor teeth (the next higher number which satisfies the rotortooth integer requirement), the calculated spacing becomes 11.45 degreeswhich is greater than the approximation, which is not acceptable.

Referring to FIG. 5, a three phase motor 90 has a stator assembly 92with 18 poles 94a-94r. As with motors 50 and 70, each stator pole haswith two teeth per each pole. Consequently there are 36 stator teeth,these being designated 96a-96aj respectively. Motor 90 includes a rotorassembly 98 disposed for rotation in a central bore 100 of the statorassembly. The rotor assembly has forty-five rotor teeth 102a-102as.Applying the formula for the number of rotor teeth: ##EQU11##

As with motors 50 and 70, motor 90 has a multiple of the required numberof rotor teeth; for example, the 45 rotor teeth 102a-102as. Applying thespacing formula: ##EQU12##

The calculated approximation between rotor teeth for motor 90 is 10.00degrees.

It will be apparent to those skilled in the art that other constructionsare possible depending upon the degree to which the stator and rotordesigns for a motor should account for the ovalizing effect on the motorassembly, the removal or elimination of cantilevered stator teeth, andthe resultant noise created as a result thereof. The various motorconstructions shown in the drawing figures are exemplary only and areused to illustrate the fact that various levels of redundancy arepossible with use of non-cantilevered teeth in a stator assembly design.Thus, three phase motors 30 and 50 exhibit a multiple of 2 for theirredundancy; while motor 70 illustrates a multiple of 2 for theredundancy in a four phase machine, and motor 90 a multiple of 3redundancy in a three phase machine.

While respect to rotor assembly construction, if the stator assembly hasonly one tooth per pole, then the rotor teeth have a step gapconstruction to improve starting torque in the desired direction ofrotation. If the stator poles have two teeth per pole, then it isimportant that the rotor have a prescribed number of required teethbased upon the above formula. In order to improve starting torque in adesired direction of motor rotation, the rotor teeth should overlap thestator teeth of other phase poles when one phase is at maximuminductance. The number of rotor teeth is limited by the ability toconstruct a stator having the appropriate calculated spacing betweenstator teeth. Thus, as indicated the spacing formula, so long as theresult of the spacing calculation exceeds approximates, and ispreferably less than that given by the formula: ##EQU13## the selectednumber of rotor teeth is the most practical for motor construction.

In view of the foregoing, it will be seen that the several objects ofthe invention are achieved and other advantageous results are obtained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:
 1. A multi-phase switched reluctance motor assembly comprising:a stator assembly having a plurality of inwardly salient teeth terminating at a central bore; and, a rotor assembly disposed for rotation in the central bore and having a plurality of opposed teeth, the stator assembly having at least one redundant pole set for each motor phase and the rotor having a corresponding proportional number of teeth the redundancy of the poles distributing the ovalizing forces on the motor assembly to lessen the effect of these forces and reduce motor noise produced by the assembly in response to such forces, the number of rotor teeth being a function of the number of stator teeth, including those for the redundant poles, and the number of motor phases, the stator assembly having a multiple number of teeth for each stator pole, and the spacing between stator poles being approximated by the formula: ##EQU14## where 360is the number of degrees in a circle, N is the number of rotor teeth, and ph is the number of phases, P/ph is the number of poles/phases, and 0.35 is a prescribed ratio of tooth width to tooth pitch.
 2. The motor of claim 1 wherein the rotor teeth are evenly spaced about the rotor assembly.
 3. The motor of claim 2 wherein the multiple is chosen such that the total number of rotor teeth exceeds the total number of stator teeth, thus to insure proper spacing between stator poles.
 4. The motor of claim 1 wherein the spacing between stator pole is approximated by the formula: ##EQU15## the desired spacing begin such that the calculated value using the formula of claim 1 is approximate to, but less than, the approximated value calculated using the formula of claim
 1. 5. The motor of claim 1 wherein none of the stator teeth are cantilevered teeth.
 6. A multi-phase switched reluctance motor comprising:a stator assembly having a plurality of stator poles; and a rotor assembly disposed for rotation relative to the stator assembly and having a plurality of rotor poles, the stator and rotor assemblies each having a single tooth per pole, and the ratio of stator poles to rotor poles being 6(n):2(n) where n is a positive whole integer having a value of 1, 2, 3, or 4 each rotor tooth having a first section defining a first air gap with respect to the stator assembly, and a second section defining a second and larger air gap with respect to the stator assembly, thereby to improve motor torque for starting the motor in one direction of rotation.
 7. The motor of claim 6 wherein each rotor tooth has a third section on the opposite side of the first section from the second section, the air gap with respect to the stator assembly defined by said third section corresponding to that defined by said second section, thereby to improve starting torque for starting the motor in either direction of rotation.
 8. The motor of claim 7 wherein the rotor teeth are evenly spaced about the rotor assembly.
 9. The motor of claim 6 wherein the spacing between stator poles is approximated by the formula: ##EQU16## the desired spacing being such that the calculated value using the formula of claim 6 is approximate to, but less than, the approximated value calculated using the formula of claim
 3. 10. A multiple phase switched reluctance motor comprising:a stator assembly having a plurality of inwardly salient teeth terminating at a central bore; and, a rotor assembly disposed for rotation in the central bore and also having a plurality of teeth, the stator assembly having at least one redundant pole set for each motor phase, and each stator pole, including the redundant poles, having a plurality of stator teeth, none of said teeth being cantilevered teeth, and the number of rotor teeth being a function of the number of stator teeth, including those stator teeth associated with the redundant stator poles, and the number of motor phases.
 11. The switched reluctance motor of claim 10 which is a three phase motor.
 12. The switched reluctance motor of claim 10 which is a four phase motor.
 13. The switched reluctance motor of claim 10 where the number of rotor teeth is a function of the number of stator teeth and motor phases.
 14. A multi-phase switched reluctance motor comprising:a stator assembly and a rotor assembly disposed for rotation relative to the stator assembly, the stator assembly and rotor assembly each having a number of poles, including redundant poles, for each motor phase, the ratio of stator poles to rotor poles being 6:2, and the stator and rotor assemblies each having a single tooth per pole, the rotor teeth each having a first section defining a first air gap with respect to the stator assembly, a second section defining a second and larger air gap with respect to the stator assembly, and a third section on the opposite side of the first section from the second section, the air gap with respect to the stator assembly defined by said third section corresponding to that defined by said second section, thereby to improve starting torque for starting the motor in either direction of rotation. 